Abstract
The causal relationships between cytosolic free-Ca2+ concentration ([Ca2+]i) increases and production of nitric oxide (NO) have been investigated mostly with indirect methods and remain unclear. Here we demonstrate, by direct real-time measurements of [NO] with a porphyrinic microsensor, that Ca2+ entry, but not an increase in [Ca2+]i, is required for triggering of NO production in human endothelial cells. Histamine, ranging from 0.1 to 100 microM, increased both NO production and [Ca2+]i when given in a single dose. However, histamine caused increased NO release but induced progressively smaller [Ca2+]i changes when cumulatively added. In the absence of a transmembrane Ca2+ gradient, no significant NO release was detectable, despite the marked Ca2+ peak induced by histamine. Inhibition of Ca2+ entry by SK&F 96365 abolished histamine-elicited NO production but only reduced the transient [Ca2+]i rise. The suppression of the sustained [Ca2+]i response under these two conditions suggests that NO release was closely associated with Ca2+ entry from the extracellular space. In addition, membrane depolarization, achieved by increasing the extracellular K+ concentration from 5 to 130 mM, reduced both the amplitude of histamine-induced sustained [Ca2+]i elevation and NO production. These results lead us to propose that the availability of numerous Ca2+ ions around the internal side of the plasma membrane would promote the association between nitric oxide synthase and calmodulin, thereby activating the enzyme.